Background & Objective: The unprecedented value and long­term uncertainties of gene therapies have challenged established health technology assessment (HTA) methods. Real-world data (RWD) and real-world evidence (RWE) may mitigate the encountered uncertainties. However, empirical evidence appears to be lacking in the literature. This research aimed to provide this evidence and identify implications for EU-wide gene therapy joint clinical assessments. Methods: An initial literature review laid the theoretical foundation for the research. A retrospective comparative analysis delivered empirical evidence on RWD/RWE usage in gene therapy HTAs. Preliminary findings were probed in three use cases and verified in semi­structured interviews. Results: Nineteen HTA reports published by the HTA bodies G­BA (Germany) and NICE (England) were identified for the ten in­scope gene therapies. Whereas NICE had an average inclusion of 14 sources per HTA report (with an acceptance rate of 56%), G­BA had 8 (with 32% acceptance rate). While RWD/RWE supporting evidence on an external comparator had the lowest acceptance rate, RWD/RWE on intervention effectiveness had the highest acceptance rate. Insufficient information to substantiate the choice of RWD/RWE and inappropriate RWE study design was the most common exclusion rationales for RWD/RWE. Conclusions: The presented empirical evidence adds to the current knowledge base in two ways; it provides empirical evidence on HTAs of gene therapies (1) and RWD/RWE usage in gene therapy HTAs (2). This work provides insights into how RWD/RWE usage differs between HTA bodies and explicates relevant considerations for gene therapy manufacturers when using RWD/RWE in future HTA submissions. It also proposes that alignment on evidentiary requirements and assessment elements will optimise the transferability of future joint clinical assessment outcomes to national HTA bodies. Knowledge transfer and knowledge sharing initiatives will play a pivotal role in achieving such a consensus.